1. Field of the Invention
The invention concerns a spacer block for an elongated substrate on the inside of an elongated tube, as well as a coaxial tube system with such a spacer block.
The invention begins with the following idea: An optimum thermal insulation for cryogenics is achieved by a multilayer foil insulation in high vacuum (superinsulation). By minimizing the components Q.sub.Gas, Q.sub.FK and Q.sub.Rad, the total heat flow is thereby brought to the lowest possible technical value so far. The heat loss flows Q.sub.Gas are reduced by creating a vacuum in the insulation space.
The solid heat bridges created by spacer blocks e.g., which are responsible for Q.sub.FK, are constructively reduced by minimizing the solid-state contact flows.
The radiation losses Q.sub.Rad are reduced by mirror-coating the internal walls or by installing highly reflective foils.
Especially with flexible, vacuum-insulated cryogenic conduits, Q.sub.FK plays a decisive role. In order for the cold internal tube not to have any direct contact with the external tube which is at room temperature, spacer blocks were used in various constructions until now. They must be able to transmit as large a force component as possible, but must have the characteristics of low thermal conductivity in the same measure. The known spacer blocks have the disadvantage that they do not attain the two required targets to the same degree. If a low thermal conductivity was attained, the mechanical load capacity was small; with a high mechanical load capacity, a very high thermal conductivity had to be taken into consideration.
2. Description of the Prior Art
A tube system made up of two concentric tubes is known from DE-C2-2 136 176, e.g. an electrical cable which operates at deep temperatures, or a tube line for transporting heated or cooled liquid or gaseous media, where the internal tube is kept in position inside the surrounding external tube by spacer blocks having a small volume or mass. The spacer blocks are spaced on the internal tube and are supported by the inside surface of the external tube. The spacer block comprises several slotted rings, where the slot width corresponds to the diameter of the internal tube. A thread element is attached to the slot area of each ring. The rings are interconnected in a way so that they can swivel around a common axis of rotation. When the spacer block is installed on the internal tube, the thread elements lay partly around the internal tube. The external tube is supported by the outside circumference of the spacer block which is formed of the rings. This construction is very small in volume or mass. However, since it is difficult to attach the thread elements to the rings permanently, and the attachment can loosen under a tension stress, this spacer block is less suited for flexible tube systems, because tension or pressure stresses of more than 10,000 N can occur when such flexible tube systems are bent.